Virtual MIMO is a simple method to realize multiple-input-multiple-output (MIMO) transmission in uplink direction (the direction from the user terminal to the base station), where each user terminal does not need at least two transmitting antennas and can only have one transmitting antenna. That is to say, virtual MIMO has the potential to increase the system capacity without any change (e.g., increase of the number of antennas) in the mobile user terminal side. It has already been defined as an option of uplink MIMO schemes in many future broadband wireless standards, such as WIMAX and 3GPP LTE etc.
The basic principle of virtual MIMO is that different user terminals (two or more) in the same cell transmit signals to the base station simultaneously using the same frequency and time resources. At the receiver (base station) side, the signals from different user terminals are treated in the same way as coming from different antennas of one user terminal. From the transmitter (user terminal) point of view, virtual MIMO doesn't lead to any change to mobile user terminals. From the receiver (base station) point of view, it is actually a spatial multiplexing (SM) scheme with each signal independently encoded and modulated, so the detection techniques for SM can be directly employed.
One major factor that restricts the performance of virtual MIMO is the unbalanced arrival power or arrival signal-to-noise ratios (SNRs) of different user terminals due to the different path losses related to the locations of the different user terminals (specifically, users carrying user terminals). With conventional detection techniques, such as maximum likelihood detection (MLD), minimum-mean-square-error (MMSE), and the QR decomposition and M-algorithm based MLD, unbalanced arrival power or arrival SNRs will seriously deteriorate the performance of virtual MIMO.
A traditional method to solve the above problem, namely, unbalanced arrival power or arrival signal-to-noise ratios (SNRs) of different user terminals is to increase the transmission power of user terminals with larger path loss through power control, so that the arrival power or arrival SNRs are the same for paired user terminals at base station side. However, this is not a power-efficient solution since from the water-filling theory, channels with poor conditions should transmit signals with small power levels in order to maximize the overall capacity. Moreover, such a kind of power control method may bring much interference to adjacent cells, which will lead to lower overall system capacity in low frequency reuse deployment.
Another way to avoid this problem is to carefully pair user terminals in virtual MIMO through scheduling so as to achieve balance of the arrival power or arrival SNRS at the base station side. However, such an approach will limit the number of user terminals who can work in virtual MIMO and consequently limit the system capacity.
Therefore, a solution that can overcome all of the above problems is needed.